Newly discovered breeding sites of Olrog's Gull Larus atlanticus in Argentina

Summary Olrog's Gull Larus atlanticus is a threatened and endemic species to the Argentine Atlantic coast. We present information on seven new colonies and update information on known colonies obtained during 1992-1995. Five of the new colonies were in southern Buenos Aires and two in southern Chubut. Total population size, including already known colonies, was estimated at 2,300 breeding pairs. All colonies were located on islands and islets and were associated with breeding Kelp Gulls Larus dominicanus. The new colonies extend the known breeding range of Olrog's Gull to the north and increase the overall known population size. However, the aerial survey also showed that two colonies found in 1990 no longer exist. Almost 40% of the total breeding population breed within the Bahia Blanca estuary, an area currently subject to intense human pressure. Olrog's Gull is still a species with restricted distribution and low population size. We suggest conservation actions needed to adequately protect this threatened species.